This disclosure relates to the field of three-phase electrical power systems and more particularly to an apparatus and method for determining the inherent system charging current of the system. Electrical arc hazards and associated affects on electrical equipment are unpredictable. Arcs may occur when equipment insulation between phase-to-phase or phase-to-ground fails. The intensity of arc damage is related to fault current magnitude at insulation failure. Power system faults data indicate that close to 75% of all faults involve ground. From this data it is clear that making the ground fault current low by using a resistance grounded power system can minimize the problem of arc hazard. The inherent system charging current value should be known for designing a high resistance (HR) grounded power system which will have lowest line-ground fault current and at the same time will avoid the occurrence of transient over voltages during such faults. There are other techniques that have been suggested by industry; however, in each case precautions need to be taken as measurements are conducted on live energized equipment. Clearly a novel measuring system for charging current is needed which does not pose a danger as compared to methods currently in use and well known. The following describes some of the contemporary techniques for determining system charging current.
In one method engineering calculations can be performed involving the inherent capacitance to ground for each of the power system components. However, this method may not provide accurate data as the sneak capacitance to ground of power system components due to their installation configuration cannot be determined.
Another approach is to use a low voltage isolated variable 120 V source voltage at the transformer neutral. This method requires that the entire power system be energized by closing the secondary breaker and keeping a fused load-interrupter switch open. The no load interrupter switch should have a bolted connection at the transformer neutral connection using an appropriate insulating boot to assure there is no arc when suddenly the load interrupter switch is closed. Initially, there should be no voltage in the bolted transformer neutral circuit when the fused load-interrupter switch is suddenly closed as the auto-transformer secondary is kept at zero volts. Then the secondary voltage of the auto-transformer is gradually increased by carefully moving the potentiometer such that there is some voltage and associated current reading. This method requires several recordings of both voltage and current to determine the constant relationship between voltage and current, which may pose danger to low voltage equipment due to the presence of medium voltage as the system is fully energized. This method poses a danger both to equipment and to personnel during measurements, and it is uncertain how the power system voltage regulation and dynamic nature of the energized power system at the time of making measurements would affect the foreign low voltage when power system voltage is present.
A further approach is to keep transformer neutral ungrounded and use a bolted connection of one phase-ground with a variable resistor and measure system charging current when the variable resistance becomes practically zero representing a bolted fault-to-ground fault on one phase. Then, using a sensitive ammeter, the direct current reading provides the system charging current. This method appears to work on the theory that when a bolted line-to-ground fault occurs on an ungrounded power system, the total system charging current will flow through the faulted phase-to-ground, and will then return to the power source through the other two phases. However, it is possible that the inductive reactance of the faulted phase conductor in series with the capacitive reactance of the un-faulted phases may develop series resonance causing dynamic over-voltage and equipment damage. Therefore, this method may pose a danger to both the equipment as well as personnel because the variable resistor needs to be changed gradually and carefully as the full line-neutral system voltage appears on the variable resistor. Changes in resistance may cause dangerous arcing on un-faulted phases as they will be at line-line voltage, especially in case of a medium voltage power system with considerable system charging current. As the system is ungrounded, dangerous transient over-voltages may occur on the ungrounded phases and cause damage.
Another approach keeps transformer neutral ungrounded and uses a bolted connection of one phase-ground to ground with a fixed resistor and a contactor to short the resistor remotely to make a bolted fault to ground and record system charging current by using a sensitive ammeter when the resistor is short circuited. This method has been discussed in the industry, but has the same drawbacks as indicated above. This is an improved method, as remote operation of the switching device, such as a contactor rated for line-to-line system voltage, can provide a short across the resistor to simulate a bolted line-to-ground fault to make direct recording of the system charging current possible. This method is again based upon the assumption that if a bolted line-to-ground fault occurs on an ungrounded power system, then the fault current will be equal to system charging current as there is no other connection of system to ground other than the capacitive coupling of the phases to ground. Again, it should be noted that this is an ungrounded power system which may cause dangerous transient over-voltages on the phases not under test when the resistor in the phase under test is short circuited due to arcing caused by system voltage regulation.